Explain or define the following terms:
$(i)$ Optical activity
$(ii)$ Polarimeter
$(iii)$ Dextrorotatory isomer

(N/A) Optical activity: When ordinary light is passed through a Nicol prism,it produces plane-polarized light. When this plane-polarized light is passed through a solution of a compound in a polarimeter,if the compound rotates the plane of polarized light,it is called an optically active compound. They are denoted as $(+)$ or $(-)$.
Polarimeter: It is an instrument used to measure optical activity. The angle of rotation of plane-polarized light is measured using this instrument.
$Dextrorotatory$ $(d)$ or $(+)$ isomer: $A$ compound that rotates the plane of polarized light to the right,i.e.,in the clockwise direction,is called a dextrorotatory or $d$-form. This rotation is indicated by a $(+)$ sign before the angle of rotation.